Digital watermarking is a process for modifying physical or electronic media to embed a hidden machine-readable code into the media. The media may be modified such that the embedded code is imperceptible or nearly imperceptible to the user, yet may be detected through an automated detection process. Most commonly, digital watermarking is applied to media signals such as images, audio signals, and video signals. However, it may also be applied to other types of media objects, including documents (e.g., through line, word or character shifting), software, multi-dimensional graphics models, and surface textures of objects.
Digital watermarking systems typically have two primary components: an encoder that embeds the watermark in a host media signal, and a decoder that detects and reads the embedded watermark from a signal suspected of containing a watermark (a suspect signal). The encoder embeds a watermark by subtly altering the host media signal. The reading component analyzes a suspect signal to detect whether a watermark is present. In applications where the watermark encodes information, the reader extracts this information from the detected watermark.
Several particular watermarking techniques have been developed. The reader is presumed to be familiar with the literature in this field. Particular techniques for embedding and detecting imperceptible watermarks in media signals are detailed in the assignee's application Ser. No. 09/503,881 (now U.S. Pat. No. 6,614,914) and U.S. Pat. No. 6,122,403, which are hereby incorporated by reference.
One application of digital watermarking is for the authentication of physical and electronic media objects, like images, video, audio, and printed media. There are a variety of ways to authenticate these objects. One way is to embed a predetermined watermark in the object. If a reader detects this watermark in an object, then the detection of the watermark is an indicator of its authenticity.
Another way to authenticate the object is to embed information about the object or the bearer of the object (e.g., in photo ID or other secure documents). If the reader extracts this information from the watermark, and it matches information on the object or about the bearer, then the comparison this information is an indicator that object is authentic and/or the bearer of the object is valid.
To undermine the authentication function of the digital watermark, a hacker might try to re-create the watermark in a fake media object.
The invention provides a method for authenticating electronic or physical media objects using digital watermarks.
One aspect of the invention is a method for creating a media object for authentication. This method computes a hash of information on the object, and generates a pattern from the hash. It also computes a content signature from a media signal in the media object. It then combines the content signature and the pattern to form a content dependent pattern. Finally, the method embeds the content dependent pattern as a digital watermark into the media object.
One specific application of this method is to create secure documents that may be authenticated automatically. For example, the media object may comprise a photo ID or other secure document, where the hash is computed from data on the document and the content signature is derived from features of the photo or other image on the document. The method applies to other physical and electronic media objects. The hash may be computed from information in the media object, which is easily interpreted by a viewer or listener of the rendered object, or may be computed from information relating to the media object.
Another aspect of the invention is a related method of authenticating a media object using a digital watermark embedded in the media object. This authentication method providing a first pattern, either from an external source (e.g., user input, system memory, etc.) or derived from a digital watermark embedded in the object. The method also derives a content dependent signature from a media signal in the media object. It then combines the content dependent signature and the first pattern to form a content dependent pattern. Finally, it measures the content dependent pattern embedded as a digital watermark in the media signal to provide a measurement of authenticity of the media signal.
Further features will become apparent with reference to the following detailed description and accompanying drawings.